The chemical name of ramosetron is (−)-(R)-5-[(1-methyl-1H-indol-3-yl)carbonyl]-4,5,6,7-tetrahydro-1H-benzimidazole, and it has the structure represented by the formula (II).

It is known that ramosetron or a salt thereof has a potent 5-HT3 receptor antagonism (Patent Reference 1, Non-patent references 1 and 2), and it is on the market as a preventive or therapeutic agent for digestive symptoms (nausea, emesis) caused by administration of an anti-malignant tumor agent (cisplatin or the like). In addition, a possibility has been reported that ramosetron or a salt thereof may be useful as an agent for treating diarrheal-type irritable bowel syndrome or an agent for improving diarrheal symptoms of irritable bowel syndrome (Patent Reference 1), and its clinical trials are now in progress as an agent for treating diarrheal-type irritable bowel syndrome or an agent for improving diarrheal symptoms of irritable bowel syndrome.
As a process for producing ramosetron or a salt thereof, the following production methods are known.
Patent Reference 1 describes a production method shown by the following Production method A, namely a method for producing a tetrahydrobenzimidazole derivative (V) by allowing a heterocyclic compound (III) to react with a carboxylic acid represented by a formula (IV) or its reactive derivative.
(Production Method A)
(In the formula, X2 is a single bond and binds to a carbon atom on the heterocyclic ring represented by Het.)
As an illustrative production method of ramosetron, Patent Reference 1 describes a production method (Production method A-1) in which racemic ramosetron are obtained by using 1-methyl-1H-indole as the compound (III), and N,N-diethyl-4,5,6,7-tetrahydrobenzimidazole-5-carboxamide or N-[(4,5,6,7-tetrahydrobenzimidazol-5-yl)carbonyl]pyrrolidine, which are acid amides, as the reactive derivative of compound (IV), and allowing them to undergo treatment with phosphorus oxychloride (Vilsmeyer reaction), and then their optical resolution is carried out by fractional crystallization using (+)-dibenzoyltartaric acid.
In addition, the Patent Reference 1 exemplifies an acid halide as one of the reactive derivatives of the compound (IV), and also describes another production method of the compound (V) (Production method A-2) in which the heterocyclic compound (III) is condensed with an acid halide of the compound (IV) by the Friedel-Crafts acylation reaction using a Lewis acid as the catalyst. However, illustrative production example of ramosetron by the Friedel-Crafts acylation reaction is not described therein.
Also, a method similar to the Production example A-1 is described in Non-patent References 1 and 2 as a production method of ramosetron.
In addition, Non-patent Reference 3 describes a method for producing ramosetron labeled with 11C, represented by a Production method B. However, it discloses only the methylation step, and does not disclose a production method of nor-YM060 as the starting material.
(Production Method B)
(In the formula, nor-YM060 means (R)-5-[(1H-indol-3-yl)carbonyl]-4,5,6,7-tetrahydro-1H-benzimidazole which was provided by the present applicant, DMF means dimethylformamide.)    Non-patent Reference 1: Chemical & Pharmaceutical Bulletin, 1996, vol. 44, no. 9, p. 1707-1716    Non-patent Reference 2: Drugs of the Future, 1992, vol. 17, no. 1, p. 28-29    Non-patent Reference 3: Applied Radiation and Isotopes, 1995, vol. 46, no. 9, p. 907-910    Patent Reference 1: JP-B-6-25153